Protective efficacy of vaccination against highly pathogenic avian influenza is dramatically suppressed by early infection of chickens with reticuloendotheliosis virus.

The present study was conducted to understand epidemiological factors that depress the protective efficacy of vaccination against highly pathogenic avian influenza (HPAI). Specific pathogen free chickens were infected at different ages with immunosuppressive reticuloendotheliosis virus (REV), then vaccinated with inactivated vaccine H5N2 and challenged with HPAI H5N1. Compared with control birds, early REV infection significantly inhibited antibody response to H5N2 vaccine and dramatically decreased protective efficacy. Immunosuppressive effects lasted for 4 months, and high mortality occurred in chickens receiving three vaccinations during 3 months prior to challenge with HPAI H5N1. Epidemiological studies indicated that REV infection and vertical transmission is common in chickens in some areas of southern China. We conclude that vaccination programmes against HPAI will not be fully effective if other immunosuppressive viral infections, such as REV, exist in chicken flocks.